I've only been to a major con once as a seller, and this wasn't an issue, mainly because everyone at my table was more or less the same level of popularity. In fact, I was kind of mooching a space off my friend's table, and I had only met them THAT DAY, and honestly I wasn't actually expecting anyone to recognize me or want artwork. (This was at F.A.C.T.S., in Ghent, the biggest yearly comic convention in the Belgium/Netherlands/Luxemburg regions. I have a small, dedicated fanbase in Belgium, but the real majority of my readers are based in the US.) Imagine my surprise when two people ACTUALLY DID know my comic and recognized my artwork. O_O They were a far cry from the sort of crowd you're talking about though.
I don't have the experience yet to know if this is possible at American cons, but I think the best way to avoid this is to plan your table with friends/related merchants in advance. It's better for everyone if you're closer to people selling related things, that way you can refer fans to each other, and more easily get them interested in each other's merch. You could even set up deals with your friends, like if you buy one book from each person you get a freebie, or something like that. That's what the girls at my table in Ghent were doing, and it was great. Rather than competing, they were all selling their stuff together and so they all had a chance to talk to interested passers-by and they also all had a chance to take breaks when they needed. It seemed like an ideal system to me.